The TN (Twisted Nematic) type display mode is now most commonly used as a liquid crystal display element. This TN type display mode has many advantages; for example, driving voltage is low and amount of electric power consumed is small. In respect of response rate, however, the TN type display mode is inferior to emissive type display elements such as a cathode ray tube, electroluminescence and plasma display. New TN type display elements in which the twisted angle is changed from 90.degree. to a range of 180.degree. to 270.degree. have been developed. These new elements, however, are still inferior in response rate. Although various attempts to improve the response rate have been made, a TN type display element sufficiently satisfactory in the response rate has not yet been obtained.
It is reported in Clark et al., Applied Phys. Lett., 36, 899 (1980) that in a new display mode utilizing ferroelectric liquid crystals which have been extensively studied in recent years, there is a great possibility that the response rate is greatly improved. This display mode utilizes a chiral smectic phase such as a chiral smectic C phase (hereafter referred to as "SC*") which exhibits ferroelectric properties. It is known that not only the SC* phase but also phases such as chiral smectic F, G, H, and I exhibit ferroelectric properties. When these ferroelectric liquid crystals are utilized as display elements, it is desired for liquid crystal material to have a ferroelectric liquid crystal phase over a wide temperature range including room temperature. At present, a single compound meeting the above requirements has not been known. Only liquid crystal compositions prepared by combining several compounds so as to satisfy the above needed characteristics as much as possible are used.
SUMMARY OF THE INVENTION
As a result of various investigations as to finding a liquid crystal compound useful as a component for the liquid crystal composition, the inventors have achieved the present invention.
Therefore, the present invention provides:
(1) a liquid crystal compound represented by the general formula (I): ##STR2## wherein R represents a straight or branched alkyl or alkoxy group having 1 to 20 carbon atoms, R* represents an optically active alkyl group having 4 to 20 carbon atoms, X represents a fluorine atom or a chlorine atom, and n is 1 or 2;
(2) a chiral smectic liquid crystal composition containing at least one liquid crystal compound represented by the general formula (I) as described above; and
(3) an optical switching element utilizing a chiral smectic liquid crystal composition containing at least one liquid crystal compound represented by the general formula (I) as described above.